1. Field of the Invention
This invention relates to a liquid crystal device having a liquid crystal provided between a pair of substrates and more particularly, to an improved liquid crystal device wherein a pair of substrates, each having a transparent electrode and an alignment film thereon in this order, are provided at a given space therebetween so that the alignment films of the respective substrates are in face-to-face relation, and a ferroelectric liquid crystal provided in the space between the paired substrates.
2. Description of the Prior Art
Studies and developments of ferroelectric liquid crystals (FLC) for application as display devices have been progressively made for the past decade. The FLC display devices have the following three characteristic features (1) to (3).
(1) High speed response (higher by 1000 times than that of known nematic display devices).
(2) Reduced dependence on an angle of view.
(3) Capability of memorizing images.
As a display technique of such ferroelectric liquid crystals, there has been proposed by Clark et al (U.S. Pat. No. 4,367,924) a surface stabilized ferroelectric liquid crystal display device wherein the cell gap between the display panels is so controlled as to be not larger than 2 xcexcm and liquid crystal molecules are aligned by use of a molecule alignment regulating force established at the interface between the panels, thereby attaining bistable energy states. Based on the high speed response in the order of microseconds and the memorizing effect of images, this device has been intensively studied and developed.
The bistable mode ferroelectric liquid crystal display has a number of features. More particularly, since the display device has the memory properties, flickering which is one of the problems of a cathode ray tube (CRT) can be avoided. Moreover, the display device can be driven at 1000 or more scanning lines by a simple X-Y matrix drive (without driving with a thin film transistor (TFT)). In addition, with regard to the problem of currently employed nematic liquid crystal displays that the viewing angle is narrow, the ferroelectric liquid crystal display has a uniform alignment of molecules and has the gap between the panels which is not greater than the half of that of the nematic liquid crystal panels, so that a wide viewing angle can be attained.
Such a ferroelectric liquid crystal display device has a structure as, for example, schematically shown in FIG. 36. More particularly, a transparent substrate 1a such as glass is provided, on which a transparent electrode layer 2a such as ITO (indium tin oxide) and a liquid crystal alignment film 3a such as, for example, a SiO oblique vacuum deposition layer are formed to provide a builtup structure A. Likewise, a substrate 1b is provided on which a transparent electrode layer 2b and, for example, a SiO oblique vacuum deposition layer 3b are formed to provide a builtup structure B. These structures are so arranged that the SiO oblique vacuum deposition layers 3a, 3b, which are, respectively, used as a liquid crystal alignment film, are facing each other. Spacers 4 are intervened between the structures to provide a liquid crystal cell. A ferroelectric liquid crystal 5 is injected into a given cell gap to complete a liquid crystal display device.
Although the FLC display device has such good features as set out hereinabove, there is the problem that an analog gray-scale display is difficult. More particularly, existing bistable mode ferroelectric liquid crystal display devices are stabilized only in two modes. Thus, it has been accepted that the devices are not suitable for the analog gray-scale display such as of video signals.
With conventional ferroelectric liquid crystal devices (e.g. surface stabilized ferroelectric liquid crystal devices), the alignment direction of the molecule M is switched between state 1 and state 2, as shown in FIG. 37, relative to an externally applied electric field E. This change in the alignment of the molecule is developed as a change in transmittance when the liquid crystal device is placed between the polarization plates which are intersected at right angles. As shown in FIG. 38, the transmittance relative to the applied electric field is abruptly changed from 0% to 100% at a threshold voltage Vth. The range or width of the threshold voltage within which the transmittance undergoes the abrupt change is generally not larger than 1 V. Accordingly, with known liquid crystal devices, it becomes difficult to have a stable threshold voltage width in the transmittance/applied voltage curve. Thus, the analog gray-scale display based on the control of the voltage will be difficult or impossible.
To cope with the difficulty, there has been proposed a a gray-scale method wherein sub-pixels are provided to appropriately control an area of pixels (pixel area gradation method), or a method wherein using high speed switching of a ferroelectric liquid crystal, the switching is repeated during one field (time integration gradation method). However, these methods have not been satisfactory with respect to the gray-scale display yet.
More specifically, with the area gradation method, an increasing number of gradations results in the increase in number of necessary sub-pixels. From the aspect of fabricating and driving display devices, it will be apparent that cost performance is not good. On the other hand, the time integration gradation method is disadvantageous in its practical utility when used alone or in combination with the area gradation method. In order to carry out an analog gray-scale display for every pixel, there has been proposed a method wherein the electric field intensity is locally graded by changing the distance between the facing electrodes within one pixel or by changing the thickness of a dielectric layer formed between the facing electrodes. Alternatively, a method has been proposed in which the voltage is graded by changing materials for the facing electrodes.
However, for the fabrication of liquid crystal display devices having analog gray-scale or gradation display characteristics at a practical level, these methods have the problem that the fabrication steps become complicated, the fabrication conditions have to be very severely controlled, and the fabrication costs become high.
On the other hand, as set forth in Japanese Laid-open Patent Application No. 3-276126, there has been proposed an FLC display device wherein the alignment film has fine alumina particles with a size of 0.3 to 2 xcexcm sprayed or distributed over the surface thereof. The area of the inversion of the ferroelectric liquid crystal domain at portions where the fine particles are present and at portions where no fine particles are present is controlled by means of an applied voltage to make an intended gray-scale display. However, with this known device, the fine particles are too large in size and the amount of the particles being sprayed is not clearly set forth. In practice, it would be almost impossible to realize an intended analog gray-scale display. More particularly, mere spraying of the fine particles with a size of 0.3 to 2 xcexcm in a cell gap of 2 xcexcm makes it very difficult to minutely change the area of the inversion of the liquid crystal within one pixel. In addition, the ferroelectric liquid crystal display device makes a display in birefringence mode of the liquid crystal, so that it will be very difficult to control the cell gap, resulting in color shading. This is considered to be the same as with existing STN (super twisted nematic) display device wherein the variation of the cell gap should be not larger than 500 angstroms.
It is accordingly an object of the invention to provide a ferroelectric liquid crystal device which overcomes the drawbacks of the prior art devices.
It is another object of the invention to provide a ferroelectric liquid crystal device which can easily, reliably realize an analog gray-scale or gradation display at low costs while keeping a high contrast.
It is a further object of the invention to provide a ferroelectric liquid crystal device wherein microdomains with different threshold voltages are developed within one pixel, so that the transmittances of the microdomains are relatively gently changed depending on the voltage being applied.
It is a still further object of the invention to provide a ferroelectric liquid crystal device wherein when liquid crystal molecules are bistable, flicker-free still images with a memory function can be formed, thereby ensuring an analog continuous gradation or gray-scale display at high contrast.
The above objects can be achieved, according to the invention, by a liquid crystal device which comprises a pair of substrates, and a liquid crystal provided between the paired substrates, domains whose threshold voltages for switching the liquid crystal are different from one another being finely distributed in the liquid crystal.